a. Field of the Invention
The present invention relates generally to fuel burning stoves, and, more particularly, to a natural draft pellet stove for heating houses and other structures.
b. Background Art
In many areas, pellet stoves for heating homes, shops, and other structures have largely superseded wood burning stoves. Pellet stoves combust pellet fuel, which is a compressed by-product of the forestry industry. The pellet fuel is conventionally made by grinding and processing bows, limbs, needles, leaves, and other waste products. By comparison with cordwood, the pellet fuel has the advantage of being more economical, and also much easier to handle and store owing to its comparatively fine consistency; commonly, pellet fuel is supplied in bags or is simply stored in a walled bin until use.
Although pellet fuel thus has many advantages that promote its use for home heating, it is not entirely an ideal fuel. In particular, because of their inherently high water and resin content, the pellets are notoriously difficult to keep lit. As a result, the majority of commercially available pellet stoves resort to the expediency of electric blowers to maintain combustion, and also use an electric auger to feed the pellets into the combustion area. These various electric motors, blowers, and feed mechanisms add substantially to the cost of the finished product, with the result that commercially available pellet stoves tend to be inordinately expensive, often to the point where they are unaffordable to many people in rural areas where they are most needed. Moreover, the cost of the electricity necessary for continuous running of the electric motors means that the electric bill for operating the pellet stove often exceeds what it would have cost to simply run an electric heater without any stove at all. Still further, the availability of electric service is somewhat spotty in some rural areas, and is subject to outages during periods of bad weather, rendering the stove inoperative just when heat is most needed.
Furthermore, reliance on the various electric blower and drive motors results in mechanical complexity and, therefore, lower reliability and higher maintenance costs; for example, it is not uncommon for conventional pellet stoves to suffer multiple blower and feed auger failures in a single season of continuous use. Also, even with the blowers to maintain the draft, the fire frequently dies out in convention pellet stoves, owing to the difficulty of keeping the fuel lit; when this happens, however, the feed auger typically continues to operate unabated, ending up packing the firebox full of unburned pellets, which may not only lead to substantial mechanical damage, but also necessitates a difficult and tedious cleanup operation to remove the packed fuel from the interior of the stove.
Perhaps even more seriously, the reliance on electric blowers leads to severe compromise of the thermal efficiency of conventional pellet stoves, so that many of these produce a dismal heat output for the amount of fuel which is consumed. In addition to inherently inefficient designs, this problem in part also stems from the tendency of manufacturers to use undersized/inadequate blowers and motors, both to cheese pare on manufacturing and also in an effort to keep operating costs down. Still further, most conventional pellet stoves lack sufficient storage capacity to operate unattended for more than a few hours before refilling, so that they are unable to keep the dwelling warm if the owner must leave for an extended period; for example, many conventional stoves are capable of holding only about xc2xc bag of pellet fuel.
Yet another problem with conventional pellet stoves is that many of these are notorious for producing excessive smoke during operation. In part, this stems again from the inability to maintain proper drafting and complete combustion of the fuel. As a result, apart from undesirable damage to the environment, pellet stoves are becoming increasingly subject to regulatory scrutiny.
Accordingly, there exists a need for a pellet stove that is capable of maintaining efficient combustion of pellet fuel using natural draft, and without the need for electric blowers to do this. Furthermore, there is a need for such a stove that is self-feeding, and does not require an auger or other electrically driven mechanism for feeding fuel into the combustion area. Still further, there exists a need for a pellet stove that ensures complete combustion of the pellet fuel so as to minimize particulates and other harmful emissions in its exhaust gasses. Still further, there is a need for such a stove that is thermally efficient, so as to produce an optimum output of heat per amount of fuel consumed. Still further, there exists a need for such a stove which is economical to manufacture, so as to be affordable for a larger group of consumers, and one which is mechanically simple and reliable so as to minimize operating and maintenance costs.
The present invention has solved the problems cited above, and is a natural draft pellet stove that sustains continuous combustion of the pellet fuel without requiring the assistance of any electrical/mechanical blowers. Broadly, this comprises: (a) feed means having a discharge opening for discharging pellet fuel, (b) grate means to which the pellet fuel is discharged for combustion from the discharge openings, (c) air supply means for providing an upward draft of combustion air through the grate means for supporting the combustion thereon, (d) exhaust means for receiving combustion gasses from the combustion of the pellet fuel on the grate means, the exhaust means having a predetermined flow capacity which is greater than a predetermined flow capacity of the air supply means so as to effectively maintain the upward draft through the grate means, and (e) means for automatically displacing the pellet fuel over the grate means away from the discharge opening as the pellet fuel is combusted, so as to keep the opening clear for discharge of additional pellet fuel onto the grate means.
The means for displacing the pellet fuel over the grate means away from the discharge opening may comprise at least one portion of the grate means having an upper surface which extends at a predetermined downward angle from the discharge opening, so that the pellet fuel rolls away from the opening during the combustion thereof; the grate means may comprise a substantially planar screen member having a sloped upper surface which forms the surface which extends at a predetermined downward angle from the discharge opening.
The predetermined flow capacity of the exhaust means may be approximately twice the predetermined flow capacity of the air supply means. The air supply means may comprise a generally horizontal air intake pipe extending from a rearward side of the stove and having a grate means mounted at a forward end thereof, so that the combustion air flows upwardly from the air intake pipe through the screen member so as to support combustion thereon.
The exhaust means may comprise first and second exhaust pipes, each exhaust pipe having an intake end positioned above and generally approximate to the screen member so that the combustion gasses generated by the combustion on the screen member flow along substantially direct paths into the intake openings, each exhaust pipe having a diameter approximately equal to a diameter of the air intake pipe. The first and second exhaust pipes may extend outwardly from their intake ends in opposite directions from one another, and the exhaust pipes may extend along an axis generally perpendicular to an axis of the air intake pipe, with the intake ends thereof being positioned substantially equidistant from the screen member at the forward end of the air intake pipe, so that the combustion gasses are substantially equally received by the exhaust pipes.
The exhaust means may further comprise first and second riser pipes mounted to the exhaust pipes so as to receive the combustion gasses therefrom, the riser pipes being connected to the exhaust pipes by elbow portions which force a flow of the combustion gasses to make a sharp directional change therein, so as to slow the flow of combustion gasses and increase the stay time thereof i the riser pipes. Each of the riser pipes preferably extends upwardly and rearwardly at a predetermined angle to vertical, so that the flow of combustion gasses therethrough maintains a rate which is selected for optimum extraction of heat therefrom as the gasses pass through the riser pipes; the predetermined angle at which the riser pipes extend may be about 40% above horizontal.
The exhaust means may further comprise at least one reburner tube mounted across the intake opening of each exhaust pipe, the reburner tube having a bore for drawing in warm air from outside the exhaust pipe and at least one cross-orifice for discharging the warm air into the flow of combustion gasses in the exhaust pipe. The reburner tube may comprise a tubular member having a central bore for drawing in the warm air and a plurality of cross-drilled bores for forming the orifices for discharging the air into the flow of combustion gasses. Preferably, there is a plurality of the reburner tubes mounted across the intake opening of each exhaust pipe.
The feed means may further comprise hopper means for storing a charge of the pellet fuel, and automatic gravity feed means for feeding the fuel in the hopper means downwardly to the discharge opening. The automatic gravity feed means may comprise at least one plate member mounted in the hopper means so as to be in contact with the pellet fuel therein. The plate member may comprise a plate member forming a directional surface sloping downwardly toward the discharge opening, and upper edge of the plate member being fixedly mounted to a framework of the stove and a lower edge being free from attachment to the framework, so that the plate member is free to distort as the member certainly expands and contracts with the changes in temperature of the stove, so as to shift the pellet fuel in the hopper means downwardly towards the discharge opening. Preferably, the at least one plate member comprises a plurality of the plate members mounted in the hopper means so as to form a downwardly sloped chute area directed towards the discharge opening, with the upper edges of the plate members being fixedly mounted to the framework of the stove and the lower ends of the plate members being free from attachment at the lower end of the chute: area, adjacent the discharge opening.
In one embodiment, the main body of the stove is formed of large diameter steel pipe, the upper part of which forms a hopper for holding several bags of pellet fuel and is closed by a hinged lid. Sloping walls feed the pellets under gravity through a small opening at the bottom of the hopper that regulates the discharge onto a stainless steel burner grate. Air is supplied from beneath the grate, through a long horizontal pipe which extends from the back of the stove and has an automatic or manual damper installed in its intake end.